两种亚热带榕属乔木切枝后的叶效率及其对生长的影响

Leaf efficiency and its role in the growth of two subtropical Ficus tree species following branch cutting

榕 ( Ficusmicrocarpa L.)和黄桷树 ( Ficus virens Ait.Var.sublanceolata( Miq.) Corner)是在三峡库区大量栽植的优良绿化树种。因种苗培育和植株管理的需要 ,对榕和黄桷树进行切枝的现象常有发生。切枝引起的叶组织损失可能使植株的总光合生产下降 ,生长减弱。因此 ,叶的净生产能力 ,即叶效率 ,对榕和黄桷树植株切枝后的生长和损伤恢复至关重要。由于切枝引起的光合组织 (叶组织 )损失大于非光合组织损失 (因为切枝并不影响植株的主茎和根组织 ) ,植株切枝后平均每单位叶面积要提供比切枝前更多的同化产物用于呼吸消耗 ,因而切枝后植株的叶效率有可能下降。为探明切枝是否会影响榕和黄桷树的叶效率以及可能的叶效率变化对植株生长的影响 ,设计了包括 2个处理季节 (春季和秋季 )和 4个处理强度 ( 0 %、2 0 %、5 0 %和 70 %切枝 )的切枝实验 ,对榕和黄桷树进行了切枝处理。实验结果表明 ,切枝不会降低榕和黄桷树的叶效率 ,相反 ,切枝后植株的叶效率会增高 (榕春季切枝处理除外 ) ,切枝对叶效率的增高效应不会因切枝季节和切枝强度的不同而变化。研究发现 ,不同切枝季节对榕和黄桷树叶效率的影响不一样。秋季切枝使叶效率增高的程度大于春季切枝。秋季切枝后 。

Ficus microcarpa L. and Ficus virens Ait. V ar. sublanceolata (Miq.) Corner are two tree species of Moraceae. Due to the ir graceful crown and capability of growing on barren fields, these two species are extensively grown in Three Gorges Reservoir area. Because of the tree manage ment and the need of cuttings for propagation (the propagation of F. microcarp a and F. virens is mainly dependent on cuttage), branch cutting occurs qui te often to these two tree species in Three Gorges Reservoir area. Branch cuttin g may lead to an overall reduction of photosynthetic production in a tree becaus e of the large loss of productive leaf tissues. Therefore, in a branch-cut tree , the net productivity of remaining leaves, i.e. the leaf efficiency of remainin g leaves, is essential to tree's regrowth and recovery. However, since branch-c utting results in a higher ratio of unproductive wood to productive leaves due t o the unaffected stem and roots, and consequently per unit area of remaining lea ves in branch-cut trees has to bear more maintenance respiration costs, whether branch-cut trees are able to achieve high leaf efficiency is uncertain. In thi s study, our aim was to investigate the leaf efficiency of branch-cut F. micr ocarpa and F. virens trees, and evaluate the role of leaf efficiency in th e growth of branch-cut trees. The study was conducted in an experimental g arden at the foot of Jinyun Mountain National Nature Reserve (29° 50′ N, 106° 26′ E). Two branch-cutting seasons (spring and autumn) and four branch-cutti ng intensities (0%, 20%, 50%, and 70% reduction of crown depth by removing branc hes) were applied in the experiment. The aboveground biomass of branch-cut tree s instantaneously after cutting as well as one year after cutting was determined indirectly based on the regression formulas constructed with harvested trees. It was shown in the experiment that branch-cutting did not decrease the leaf ef ficiency of both F. microcarpa and F. virens trees. Conversely, the leaf efficiency of F. microcarpa and F. virens trees was increased after bra nch cutting, regardless of the intensity and the season of branch-cutting (exce pt that the leaf efficiency of spring-treated F. microcarpa trees did not a lter too much). It was found that the effects of branch-cutting on the increase of leaf efficiency varied with cutting season. For both species, branch-cuttin g conducted in autumn led to a larger increase in leaf efficiency than that cond ucted in spring. Furthermore, the pattern of leaf efficiency changing with cutti ng intensity differed between cutting seasons. The leaf efficiency of autumn-cu t trees increased with cutting intensity, but the leaf efficiency of spring-cut trees did not present this pattern. The experimental results revealed that incr eased leaf efficiency contributed to the aboveground biomass gain of F. microc arpa and F. virens trees after branch cutting. The proportions of abovegro und biomass gain due to the increase of leaf efficiency to the overall abovegrou nd biomass gain one year after cutting were 7.9%, 12.9%, and 21.6% in 20%, 50%, and 70% autumn-cut F. microcarpa trees; 24.4%, 18.1%, and 15.4% in 20%, 50% , and 70% spring-cut F. virens trees; and 9.5%, 19.9%, 35.3% in 20%, 50%, a nd 70% autumn-cut F. virens trees. Undoubtedly, the increase of leaf effici ency following branch-cutting could, to some extent, alleviate the reduction of photosynthetic assimilate production of branch-cut trees. Raising leaf efficie ncy is a useful strategy of trees to mitigate the adverse effects of external da mages on growth.